Prior art fire and smoke detectors can generally be grouped into three types: ionization gauges, heat detectors and optical smoke detectors. An ionization gauge has an ionization chamber, wherein a small quantity of americium-241 is used to provide a source of alpha particles. Alpha particles constantly released by the americium knock electrons off of atoms in the air, thereby ionizing the oxygen and nitrogen atoms in the chamber. These ionized atoms and electrons generate a small, continuous electric current via two electrode plates. When smoke enters the ionization chamber, the smoke particles attach to the ions and neutralize them, so some of them do not reach the plates. Consequently, the drop in the electric current triggers the alarm. The ionization gauge is not suitable for use in a small electronic device, such as a mobile phone, because the gauge requires a sufficiently large ionization chamber to produce a useful electric current. Furthermore, although alpha particles do not normally post radiation risks, a mobile phone user may find it uncomfortable to expose the head and ear to a radioactive source at close range. Also, delivering, storing and handling a large quantity of radioactive raw material for manufacturing may not be a viable option for a cell phone manufacturer.
Heat detectors only measure unusually large and sudden increases in room temperature. By the time a heat detector detects such an increase to make a warning, it would generally be too late to protect life and property in the room, because the room may be already filled with heavy smoke and flames. Furthermore, a telephone set may generate sufficient amount of heat during a call or during a video clip viewing session. The heat so generated may be indistinguishable from the heat generated by a fire.
Prior art optical smoke detectors usually rely on two main configurations: forward light scattering and backward light scattering in a dark chamber. Herwig et al. (U.S. Pat. No. 4,397,557) discloses a smoke detector wherein a light emitter and a light detector are arranged in a dark chamber having a plurality of light barriers or diaphragms such that the light detector normally does not see a significant amount of light in the dark chamber. The light beam from the light emitter is confined to a light transmission path or volume. Likewise, the light detector is able to detect light within a light reception path or volume. The light transmission path and the light reception path intersect at a location referred herein as a cross-path area, as shown in FIG. 1a. When smoke particles drift into the cross-path area, they scatter the light emitted from the light emitter and part of the scattered light is detected by the light detector. The relative orientation of the light emitter and the light detector is such that the scattering angle, β, is smaller than 90°, as shown in FIG. 1b. Thus, the smoke detection scheme, according to Herwig et al., is based on the backscattering of light by smoke particles in a dark chamber.
Smoke detection can also be based on forward scattering of light by smoke particles in the dark chamber. As shown in FIG. 2a, a dark chamber having labyrinths (not shown) is used to prevent ambient light from being detected by the light detector. Thus, only air, but not ambient light, can enter the dark chamber. At the same time, a light stop is used to prevent the light detector from seeing the light beam from the light emitter directly. However, when smoke particles drift into the cross-path area in the dark chamber, the scattered light by the smoke particles can be detected by the light detector. As shown in FIG. 2b, the scattering angle is greater than 90°.
The prior art smoke detectors based on backward or forward scattering of light require a dark chamber to shield ambient light from entering the smoke-detection area. Such a smoke detector cannot be practically implemented on a small portable device, such as a mobile phone, because it requires a dark chamber.
It is thus advantageous and desirable to provide a method and device for smoke detection using opto-electronic components that can be disposed on a mobile phone without a dark chamber.